In accordance with increase in data traffic, a high-capacity trunk light communication network has been demanded, and high-speed communication of 40 Gbps (Giga bit per second) or 100 Gbps are putting into practice. For high-speed communication in an optical communication network, one of the techniques recently focused is optical transmission and optical reception using digital signal processing.    [Non-Patent Literature 1] D.-S. Ly-Gagnon, et al. “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation”, IEEE JLT, vol. 24, no. 1, pp. 12-21, January 2006    [Non-Patent Literature 2] D. McGhan, “Electronic Dispersion Compensation”, OFC2006, OWK1
In general, a wavelength locker having, for example, an etalon filter is used to stably output light having a constant oscillation frequency (wavelength) from a transmitting light source used in a wavelength multiplexed optical transmission system. Considering aged deterioration, the accuracy of control by such a wavelength locker is several GHz. Fluctuation of this several GHz causes lowering the transmission capability in an optical transmission system and hinders high-density intervals of wavelengths to be multiplexed in a single optical fiber. In other words, frequency stability of a transmission light source is factors limiting the transmitting distance in an optical transmission system and a transmission capacity of a single optical fiber.
An oscillation frequency (wavelength) of a transmission light source is generally controlled by temperature, which takes time to make it difficult to flexibly change the wavelength arrangement particularly during the operation of the optical transmission system. More specifically, such control may take several minutes and the oscillation frequency to be controlled may have poor stability. In addition, it is difficult to accomplish complex and precise control while the system is working.